Numerous varieties of plating technologies are known in the art. These technologies include electrolytic plating which is also known as electro-plating and by other terms, and electroless plating also known as chemical, autocatalytic and by other terms.
Electroless plating is a well known and established commercial/industrial process for metal plating. The metal portion of the metal salt may be selected from suitable metals capable of being deposited through electroless plating. Such metals include, without limitation, nickel, cobalt, copper, gold, palladium, iron, other transition metals, and mixtures thereof, and any of the metals deposited by the autocatalytic process described in Pearlstein, F., “Modern Electroplating”, Chapter 31, 3rd Ed., John Wiley & Sons, Inc. (1974), which is incorporated herein by reference. Generally, the electroless metal in the deposited coating is a metal, a metal alloy, a combination of metals, or a combination of metals and non-metals. Such coatings are often in the form of a metal, a metal and phosphorous, or a metal and boron. The metal or metal alloy is derived from the metal salt or metal salts used in the bath. Examples of the metal or metal alloy are nickel, nickel-phosphorous alloys, nickel-boron alloys, cobalt, cobalt-phosphorous alloys, and copper alloys. Other materials such as lead, cadmium, bismuth, antimony, thallium, copper, tin, and others can be deposited to form the bath and included in the coating.
The salt component of the metal salt may be any salt compound that aids and allows the dissolution of the metal portion in the bath solution. Such salts may include without limitation, sulfates, chlorides, acetates, phosphates, carbonates, and sulfamates, among others.
The reducing agents are electron donors. When reacted with the free floating metal ions in the bath solution, the electroless reducing agents reduce the metal ions, which are electron acceptors, to metal for deposition onto the article. The use of a reducing agent avoids the need to employ a current, as required in conventional electroplating. Common reducing agents are sodium hypophosphite, sodium borohydride, n-dimethylamine borane (DMAB), n-diethylamine borane (DEAB), formaldehyde, and hydrazine.
Certain materials may be used in electroless plating baths where these materials serve two or more roles in the plating bath. For example, instead of using the typical combination of nickel sulfate as a metal salt and sodium hypophosphite as a reducing agent, it is possible to use nickel-hypophosphite in an electroless nickel plating bath. Nickel-hypophosphite, however, is very expensive and not widely used commercially due to its impractical cost.
Electroless nickel (EN) is one of the most commercialized varieties of electroless plating. It is an alloy of nominally 86-99% nickel and the balance with phosphorous, boron, or a few other possible elements. Electroless nickel is commonly produced in one of four alloy ranges: low (1-5% P), medium (6-9% P), or high (10-14% P) phosphorous, and electroless nickel-boron with 0.5-5% B. Each variety of electroless nickel thus provides properties with varying degrees of hardness, corrosion resistance, magnetism, solderability, brightness, internal stress, lubricity, and other properties. All varieties of electroless nickel can be applied to numerous articles, including metals, alloys, and nonconductors.
Electroless composite technology is a more recent development as compared to electrolytic composite technology. The fundamentals of composite electroless plating are documented in a text entitled “Electroless Plating Fundamentals and Applications,” edited by G. Mallory and J. B. Hajdu, Chapter 11, published by American Electroplaters and Surface Finishers Society (1990).
The plating of articles with a composite coating bearing finely dispersed divided particulate matter is well documented. The inclusion of finely divided particulate matter within metallic matrices can significantly alter the properties of the coating with respect to properties such as wear resistance, lubricity, friction, thermal transfer, and appearance.
The co-deposition of particles in composite electroless plating can dramatically enhance existing characteristics and even add entirely new properties. These capabilities have made composite electroless coatings advantageous for a variety of reasons including, but not limited to, increased utility in conditions requiring less wear, lower friction, lubrication, indication, authentication, thermal transfer, insulation, higher friction, and others. Composite electroless coatings with nickel provide an additional environmental advantage over conventional electroless nickel coatings, which do not include particulate matter, in that the particles within composite electroless nickel coatings reduce the amount of nickel alloy used. Such nickel based composite coatings are also an alternative to chromium based coatings which pose certain health and environmental challenges.
Particulate matter suitable for practical composite electroless plating may be from nanometers up to approximately 75 microns in size. The specific preferred size range depends on the application involved.
The particulate matter may be selected from a wide variety of distinct matter, such as but not limited to ceramics, glass, talcum, plastics, diamond (polycrystalline or monocrystalline types, natural or manmade by a variety of processes), graphite, oxides, silicides, carbonate, carbides, sulfides, phosphate, boride, silicates, oxylates, nitrides, fluorides of various metals, as well as metal or alloys of boron, tantalum, stainless steel, molybdenum, vanadium, zirconium, titanium, tungsten, as well as polytetrafluoroethylene (PTFE), silicon carbide, boron nitride (BN), aluminum oxide, graphite fluoride, tungsten carbide, talc, molybdenum disulfide (MoS), boron carbide and graphite. The boron nitride (BN), without limitation, may be hexagonal or cubic in orientation.
For increased friction on the surface of a resultant coating and/or increased wear resistance, hard particulates, such as but not limited to diamond, carbides, oxides, and ceramics, may be included in the plating bath. Application of an overcoat of a conventional plated layer on top of the composite plated layer is also done in the field in order to further embed the particulate matter within the coating.
For increased lubrication or reduction in friction in the resultant coating, “lubricating particles,” such as polytetrafluoroethylene (PTFE), boron nitride (BN), talc, molybdenum disulfide (MoS), graphite or graphite fluoride among others may be included in the plating bath. These lubricating particles may embody a low coefficient of friction, dry lubrication, improved release properties, and/or repellency of contaminants such as water and oil.
For light emitting properties in the resultant coating, particulates with phosphorescent properties such as, but not limited to, calcium tungstate may be included in the plating bath.
For identification, authentication, and tracking properties in the resultant coating, various particulate and solid materials may be included in the plating bath so they will be incorporated into the coating and detectable either visually, under magnified viewing, or detection with a suitable detector.
The inclusion of insoluble particulate matter in composite electroless baths introduces additional instability. To overcome the extra instability due to the addition of insoluble particulate matter to the bath, such as described in U.S. Pat. No. 6,306,466, the general use of particulate matter stabilizers (PMSs) is believed to isolate the finely divided particulate matter, thereby maintaining the particular matter's “inertness”. Such PMSs are well-known, and include, without limitation, sodium salts of polymerized alkyl naphthalene sulfonic acids, disodium mono ester succinate (anionic and nonionic groups), fluorinated alkyl polyoxyethylene ethanols, tallow trimethyl ammonium chloride, and any of the PMS disclosed in U.S. Pat. No. 6,306,466, which is incorporated herein by reference.
The electroless metallizing bath may also contain one or more complexers, also known as complexing agents. A complexing agent acts as a buffer for reasons which may include pH control and maintaining control over the “free” metal salt ions in the solution, all of which aids in sustaining a proper balance in the bath solution.
The electroless metallizing bath may further contain a pH adjuster to also help control pH levels in the bath. Suitable pH adjusters may buffer the plating bath at a desired pH range.
Some materials may serve one or more functions within an electroless plating bath. For example, ammonium hydroxide is both a pH adjuster as well as a complexer; cadmium, aluminum, copper and others materials are both a stabilizer and a brightener, lactic acid is both a complexer and a brightener, some sulfur compounds like thiourea are both stabilizers and accelerators depending on concentration, and there are other multipurpose ingredients useful in electroless plating baths.
Ingredients typical in electroless plating and useful in the present invention include, but are not limited to the following materials in the following general categories:
Complexers
Acetic Acid, Alanine-beta, Aminoacetic Acid, Ammonium Bicarbonate, Ammonium Carbonate, Ammonium Chloride, Ammonium Hydroxide, Boric Acid, Citric Acid, Citrates, EDTA, Ethylenediamine, Fluoboric Acid, Glycerine, Glycine, Glycolic Acid, Glycolic Acid Salts, Hydroxyacetic Acid, Lactic Acid, Maleic Anhydride, Malic Acid, Malonic Acid, Orthoboric Acid, Oxalic Acid, Oxalic Acid Salts, Propionic Acid, Sodium Acetate, Sodium Glucoheptonate, Sodium Hydroxyacetate, Sodium Isethionate, Sodium or Potassium Pyrophosphate, Sodium Tetraborate, Succinic Acid, Succinate Salts, Sulfamic Acid, Tartaric Acid, Triethanolamine, Monocarboxylic Acids, Dicarboxylic Acids, Hydrocarboxylic Acids, Alkanolamines, and combinations and variations of such materials.
Stabilizers
2 Amino-Thiazole, Antimony, Arsenic, Bismuth Compounds, Cadmium Compounds, Lead Compounds, Heavy Metal Compounds, lodobenzoic Acid, Manganese Compounds, Mercury Compounds, Molybdenum Compounds, Potassium Iodide, Sodium Isethionate, Sodium Thiocyanate, Sulfur Compounds, Sulfur Containing Aliphatic Carbonic Acids, Acetylenic Compounds, Aromatic Sulfides, Thiophenes, Thionaphthalenes, Thioarols, Thiodipropionic Acid, Thiodisuccinic Acid, Tin Compounds, Thallium Sulfate, Thiodiglycolic Acid, Thiosalicylic Acid, Thiourea, and combinations and variations of such materials.
Brighteners
Aluminum, Antimony Compounds, Cadmium Compounds, Copper, Lactic Acid, and combinations and variations of such materials.
pH Controllers
Ammonium Bicarbonate, Ammonium Carbonate, Ammonium Chloride, Ammonium Hydroxide, Potassium Carbonate, Potassium Hydroxide, Sodium Hydroxide, Sulfamic Acid, Sulfuric Acid, and combinations and variations of such materials.
Particulate Matter Stabilizers (Dispersants, Surfactants, Wetters)
Sodium salts of polymerized alkyl naphthalene, disodium mono ester succinate (anionic and nonionic groups), fluorinated alkyl polyoxyethylene ethanols, tallow trimethyl ammonium chloridesulfonic acids, disodium mono ester succinate (anionic and nonionic groups), fluorinated alkyl polyoxyethylene ethanols, tallow trimethyl ammonium chloride, and any of the PMS disclosed in U.S. Pat. No. 6,306,466, which is incorporated herein by reference, and combinations and variations of such materials.
Buffers
Borax, Boric Acid, Orthoboric Acid, Succinate Salts, and combinations and variations of such materials.
Reducing Agents
DMAB, DEAB, Hydrazine, Sodium Borohydride, Sodium Hypophosphite, and combinations and variations of such materials.
Accelerators
Fluoboric Acid, Lactic Acid, Sodium Fluoride, Anions of some mono and di carboxylic acids, fluorides, borates, and combinations and variations of such materials.
Metal Salts
Cobalt Sulfate, Copper Sulfate, Nickel Sulfate, Nickel Chloride, Nickel Sulfamate, Nickel Acetate, Nickel Citrate, and combinations and variations of such materials.
Historically electroless nickel and composite electroless plating processes have included heavy and/or toxic metals in the plating bath to overcome the inherent instability of the plating bath. Lead has been the most commonly used material to serve this purpose. Cadmium has also been used widely over the years as a brightener for electroless nickel coatings. But this incorporation of heavy metals into the plating baths presents multiple challenges. The heavy metals must be added in a sufficient amount to prevent the decomposition of the plating bath, but an increased concentration beyond the necessary level required to prevent the decomposition results in cessation or reduction of the plating rate. Increasingly stringent rules and regulations that restrict or prohibit the use of heavy metals, such as the Removal of Hazardous Substances (RoHS) and End-Of-Life Vehicle (ELV) Regulations. However, U.S. Pat. Nos. 7,744,685 and 8,147,601 disclose stable composite electroless nickel plating baths without the use of heavy and/or toxic metals. These patents are included herein by reference.
The electroless nickel and composite electroless nickel solutions of the present invention may contain heavy metals or may be essentially free of heavy metals, which means that no such heavy metal is added to the plating bath and/or the heavy metal concentration should be no more than a level that would cause the coating on articles plated in said bath to have a heavy metal concentration in excess of any relevant regulations. The solutions of the present invention may also contain heavy metals less toxic and/or subject to fewer regulations than lead, cadmium and others.
In recent years, there has been a growing desire within the plating industry to avoid the use of ammonium hydroxide. Ammonium hydroxide is an effective complexing agent and pH adjuster. Ammonium hydroxide, however, is objectionable to some plating shops due to environmental, health and/or safety regulations, smell, and the difficulty it causes in the ability to remove the nickel from the plating bath at the end of the bath's life because it is such a strong complexing agent. Storage and handling of ammonium hydroxide is also problematic as it can cause storage drums and other containers to bloat, it emits a very noxious odor experienced when opening a container, pumping, and transporting ammonium hydroxide, and causes a strong reaction when added to a hot plating bath unless the extra step of diluting the ammonium hydroxide by 50 percent by volume or more is performed in advance. Specially designed respirators are needed when handling ammonium hydroxide. It is therefore desirable to have a solution for an electroless nickel plating bath where this solution is free of ammonium hydroxide, and whereby the user or plater has the ability to use a material other than ammonium hydroxide as an auxiliary solution to maintain the pH of the plating bath during usage. The present invention is able to operate effectively with or without ammonium hydroxide. The present invention is able to operate effectively with sodium hydroxide, potassium hydroxide, potassium carbonate, and the like as pH adjusters within the solution of the present invention or as auxiliary additives to affect the pH of the plating bath made with the solution of the present invention.
In recent years, there has been a growing desire within the plating industry to use lower concentrations of metal salts in the plating baths. The primary justifications for this alternative to the conventional concentrations of metal salts in the plating baths are to 1) reduce the drag out of the metal salts from the plating baths to the subsequent rinse tanks and thereby reduce the amount of metal salts that need to be captured in subsequent waste treatment of the rinse water facilitating better environmental practices, 2) reduce the amount of metal salts that are essentially wasted when the plating bath comes to the end of its useful life and the bath is waste treated or otherwise disposed of, and 3) improve the quality of the plating by lowering the amount of metal salts in the bath which have the potential to precipitate or react in the bath in ways other than the desired reduction and deposition onto articles immersed in the plating bath for the purpose of plating, especially effective in reducing shelf roughness, 4) lowering the cost to make up a plating bath, 5) extend plating bath life, especially when plating onto aluminum substrates, 6) increase reducing agent efficiency, and 7) contain less metal and other substances in the mist emanating from the plating bath. An example of this practice is in the electroless nickel plating field where some platers are using plating baths with less than the traditional 6 grams per liter of nickel metal in the bath, for example, 3 grams per liter. The background and justification for using electroless nickel plating baths with a reduced nickel content is well documented in: www.pfonline.com/articles/fifth-generation-reduced-ion-electroless-nickel-systems. When applied to electroless nickel plating systems, the present invention is able to operate effectively at a traditional concentration of 6 grams per liter of nickel metal in the plating bath, 3 grams per liter of nickel metal in the plating bath, and other concentrations. Formulation of the solution useful for make up and replenishment of an electroless nickel plating bath according to the present invention, but using less than the amount of a metal salt required to yield the traditional 6 grams per liter of nickel metal in the plating bath, has the benefit of reducing the quantity of ingredients in the solution and thereby making the solution easier to formulate and concentrate.
In addition, in recent years, health and environmental concerns have been raised about the inclusion of certain materials such as perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) that may be used in plating systems including composite plating systems, including those with PTFE. PFOS may be contained in certain particulate matter stabilizers (PMSs) useful for electroless plating. The present invention therefore includes compositions, baths, and methods for plating that may contain PFOA and/or PFOS, or may be free, or have only trace amounts of PFOA and/or PFOS.
While many elements of the EN plating chemistry, process, and industry have evolved, one essential aspect of the technology has remained relatively unchanged since the early style baths were surpassed by formulations that were easier and more reliable to operate. This aspect is the method to make up and maintain the EN plating bath. Make up of an EN bath involves combining the ingredients required to create a bath that is ready to be used for its intended purpose. Maintenance or replenishment of the EN bath involves replacing the chemical elements of the bath that have been depleted from the bath as plating occurs from the bath onto articles immersed in the bath.
While it is possible to make up and replenish a plating bath by adding the desired amount of each individual ingredient to form a solution, the established method to make up and replenish a plating bath is to combine three or more separate pre-made solutions with water.
When three solutions are used, it is common in the field to make up an EN bath with an “A” solution and a “B” solution and water. The A solution typically contains the metal salt (for example, nickel sulfate), may contain other ingredients, and accounts for five to six percent of the volume of the plating bath. The B solution typically contains the reducing agent (for example, sodium hypophosphite), other functional ingredients like stabilizers, brighteners, pH buffers, chelators, complexing agents, accelerators, particulate matter stabilizers, etc., and accounts for fifteen to twenty percent of the volume of the plating bath. The balance, typically about eighty percent of the volume of the plating bath, is made up of water plus the possibility of an acid or base to adjust the pH of the EN bath before it is heated to the desired temperature and used for plating. The water is typically deionized water. That is, the initial bath is comprised of the A solution, the B solution, water, and potentially a pH adjuster, where the pH adjuster may be introduced into the water before being combined with A and B.
The use of multiple plating compositions as described herein, is referred to as a “plating bath system”.
As the bath is used, it needs to be replenished. The EN bath is then typically replenished with the A solution as well as a “C” solution. The C solution is typically similar to the B solution, containing the reducing agent (for example, sodium hypophosphite), other functional ingredients like stabilizers, brighteners, pH buffers, chelators, complexing agents, accelerators, particulate matter stabilizers, etc., but the specific combination and concentration of these materials are in different concentrations in the C solution than they are in the B solution. The reason for the difference of concentrations of these materials is the difference in the consumption or depletion rate of each material from the initial make up concentration due to the plating reaction. The C solutions are typically formulated to be used in a convenient ratio to the A solutions, for example one part A solution plus two parts C solution; or for example one part A solution plus one part C solution.
When more than two solutions are used, such as the Addplate™ concentrate systems sold by Surface Technology, Inc. of Trenton, N.J., it is common in the field to make up an EN bath with three solutions such as 1) an “M” solution, 2) a solution of nickel sulfate, and 3) a solution of sodium hypophosphite, plus water. The M solution typically contains the functional ingredients like stabilizers, brighteners, pH buffers, chelators, complexing agents, accelerators, particulate matter stabilizers, etc., and accounts for eight to ten percent of the volume of the plating bath. The nickel sulfate and sodium hypophosphite solutions typically account for four and a half percent each of the volume of the plating bath. The balance, typically about eighty-two percent of the volume of the plating bath, is made up of water plus the possibility of an acid or base to adjust the pH of the EN bath before it is heated to the desired temperature and used for plating. The water is typically deionized water. The EN bath is then typically replenished with an “R” solution as well as the nickel sulfate and sodium hypophosphite solutions. The R solution is typically similar to the M solution, containing the functional ingredients like stabilizers, brighteners, pH buffers, chelators, complexing agents, accelerators, particulate matter stabilizers, etc., but the specific combination and concentrations of these materials are in different concentrations in the R solution than they are in the M solution. The reason for the difference of concentrations of these materials is due to the difference in the consumption or depletion rate of each material from the plating bath during usage of the plating bath and the plating reaction. The R solutions are formulated to be used in a convenient ratio to the nickel sulfate and sodium hypophosphite solutions, for example one part nickel sulfate solution plus one part sodium hypophosphite solution plus one part R solution; or for example one part nickel sulfate solution plus one part sodium hypophosphite solution plus one half or one third part R solution.
Some companies in the plating industry have offered and/or used systems where the bath can be made up of one single component instead of two, three or more. But in none of these systems is it possible to replenish that same bath with the same make up solution for ongoing maintenance of the bath over the bath's life while providing proper bath stability and plating quality.
It is possible, especially as would be evident to one skilled in the art from understanding the present invention, to operate an electroless plating bath with one component used alone to make up the plating bath and a second component used alone to replenish the plating bath. Such a two component system still lacks the full utility of the single component of the present invention.
When discussing the materials and solutions used in the make up and replenishment of electroless plating baths, and if the system is a one, two, three, four or more solution system, it is customary in the field to count the number of solutions containing the primary functional ingredients such as metal salts, reducing agents like stabilizers, brighteners, pH buffers, chelators, complexing agents, accelerators, particulate matter stabilizers, etc., and mixtures thereof. The addition of any other ingredients to the plating bath is not considered an additional solution. For example, the addition of materials such as ammonium hydroxide, other hydroxides, carbonates and the like to adjust the pH of the plating bath are not considered a solution in the same way as a typical A, B, C, M or R solution is counted in the system. These materials are considered auxiliary solutions. Solutions of additional stabilizers, brighteners, accelerators, PMSs, and other materials may also be used as auxiliary solutions to modify the plating bath for specific purposes, often for episodic purposes rather than consistent uses. If such materials were needed for consistent, routine purposes in the plating bath, they might be incorporated into one or more of the primary solutions such as the A, B, C, M or R solutions. Similarly, the addition of particulate matter, in powder, liquid dispersion, or other form, is also considered an auxiliary material or solution, and is not considered a solution or component in the same way as a typical A, B, C, M or R solution is considered as a solution in the system.
Consequently, it would be beneficial for a single solution usable for both initial and replenishment purposes.
The typical operation of an electroless plating bath consists of the following steps. First, a plating bath is made up traditionally as already discussed in this disclosure. The plating bath is then heated by any of a number of mechanisms to reach a desired operating temperature. Articles for plating are then cleaned and otherwise pretreated according to their base metal(s) and condition, and immersed into the plating bath. While the articles are being plated for a time commensurate with the plating rate of the plating bath and the desired thickness of the plating onto the articles, the temperature and pH of the plating bath are typically monitored and maintained at desired levels. During or after the plating of the articles, the plating bath is analyzed to determine the amount of certain components in the plating bath. Typically this analysis is only for the metal of the metal salt in the plating bath, and this is accomplished by wet chemistry or by instrumental analysis. Based on the concentration of this metal in the plating bath, the plating bath is traditionally replenished with two or more solutions containing the ingredients needed to replace what has been depleted onto the articles. This replenishment can be added to the plating bath by pouring, pumping, or other means. Analysis of other components such as reducing agents and stabilizers in the plating bath can be accomplished, but is much less common, and therefore increases the potential for the ratio of ingredients to become imbalanced with the metal salt and other ingredients in the plating bath. This represents one further advantage of the present invention whereby the ingredients will remain in the proper ratio as they are all contained in the single primary component used for make up and replenishment of the plating bath.